The subject matter of the present invention relates to a perforating method and apparatus, and more particularly, to a perforating method wherein a perforating gun is lowered into a well to a first depth on tubing, subsequently lowered further into the well to a second depth on wireline, anchored to the well casing, and all wireline apparatus is disconnected from the perforating gun and withdrawn from the wellbore prior to performing a standalone perforation operation.
Various techniques have been utilized for perforating a well casing. One such technique is disclosed in U.S. Pat. No. 4,349,072 to Escaron et al. This technique involves lowering tubing into a borehole, such as a deviated well, the tubing including a well instrument, such as a perforating gun, and subsequently lowering the well instrument further into the borehole via wireline. When the instrument is lowered to its desired location in the well, on wireline, the well instrument is activated. In the case of a perforator, the perforator is discharged into the formation. Another similar technique, although not involving a perforator, is disclosed in U.S. Pat. No. 4,690,214 to Christian Wittrisch. In the Wittrisch patent, a tubing including a well instrument is lowered into the well, the well instrument being subsequently lowered into the well via wireline. The instrument is anchored to the well casing, and the wireline tension is reduced, prior to performing a measurement function. Although the well instrument is not disclosed as being a perforator, the wireline remains attached to the well instrument during the measurement function. When the well instrument is a perforating gun, in hot, deep wells, after the perforating gun is lowered into the well on wireline, it is not desirable that the wireline remain connected to the perforating gun. If the wireline remains connected to the gun, it must be sealed off at the surface during perforation to provide for safe pressure control. This is accomplished by using a lubricator and a riser, the lubricator containing many seals and connections. In addition, if the wireline remains connected to the gun when the well produces, the wireline and other tools must subsequently be retrieved from the well against significant well fluid pressure. Furthermore, if the wireline remains connected to the gun, during perforation, the wireline may accidentally disconnect from the gun and blow upwardly toward the surface of the well thereby creating a "birdsnest"; as a result, an expensive fishing operation would be required for untangling the wireline and retrieving the perforating gun. In hostile environments, such as H.sub.2 S, the wireline may be damaged if it remains in the borehole for long peroids of time. In addition, if the wireline remains connected to the gun, the wireline itself may represent an obstruction with respect to unrestricted flow o well fluid from the perforated openings in the formation to the well surface. It is more desirable that the perforating gun "standalone" in the well, that is, that it be anchored to the well casing, and all wireline be withdrawn to the well surface prior to discharging the perforating gun into the formation. As a result, an unrestricted flow of well fluid toward the surface is obtained. In addition, a safer perforation operation is performed, since there is no wireline to obstruct or otherwise complicate the perforation operation. Since a wireline is not connected to the gun, a simple master valve may be provided below the lubricator for surface pressure control. The master valve provides for safe operation and it minimizes the amount of perforating equipment components utilized downhole.